Maintenance (weeks 30-101): Patients receive oral mercaptopurine daily on days 1-28 and 36-56; oral methotrexate weekly; and nelarabine IV on days 29-33. Treatment repeats every 8 weeks for 4 courses. Beginning on week 62, patients receive vincristine IV once; oral prednisone three times daily for 5 days; oral mercaptopurine daily; and oral methotrexate weekly. Treatment repeats every 8 weeks for 5 courses.